12 research outputs found
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SNM measurement uncertainties: Potential impacts for materials disposition
A brief discussion of various issues relative to nuclear measurement uncertainties and impacts to the Materials Disposition (MD) program is presented. Today`s nuclear measurement technology is well situated to handle most of materials analysis concerns while controlling uncertainties to a high degree of confidence. However many of the options under consideration by the disposition program will present new challenges. Some of these challenges include significant material processing throughputs, a variety of material forms, unique waste streams, and difficult to measure matrices. There are also some questions as to a facility`s ability to achieve IAEA verification requirements and to maintain measurement uncertainties within the significant quantity level
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The development of a virtual heat bath for calorimeters
All existing calorimeter systems for sensitive nuclear assay employ a heat bath surrounding the sample chamber. The purpose of the heat bath is to maintain a constant temperature so that a fixed temperature difference is maintained across the thermal resistance of the calorimeter. Present calorimeter systems all employ an active, feedback-controlled system to maintain a fixed temperature. An alternative would be to allow the heat-bath temperature to change, to measure it, and to compensate the assay for this change. Two significant observations make this approach possible: (1) the effect on the measurement of a temperature change in the heat bath is differential in form and (2) temperature measurement systems are very accurate when measuring differences in temperature (either in time or between two locations). From these observations, the authors have developed a virtual heat-bath compensation system. The control theory and results will be presented
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High fluence neutron source for nondestructive characterization of nuclear waste. 1998 annual progress report
'The author is addressing the need to measure nuclear wastes, residues, and spent fuel in order to process these for final disposition. For example, TRU wastes destined for the WIPP must satisfy extensive characterization criteria outlined in the Waste Acceptance Criteria. Similar requirements exist for spent fuel and residues. At present, no nondestructive assay instrumentation is capable of satisfying all of the requirements. One of the primary methods for waste assay is by active neutron interrogation. The authors plan to improve the capability of all active neutron systems by providing a higher intensity neutron source (by about a factor of 1,000) for essentially the same cost, power, and space requirements as existing systems. This high intensity neutron source will be an electrostatically confined (IEC) plasma device. The IEC is a symmetric sphere that was originally developed in the 1950s as a possible fusion reactor. It operates as D-T neutron generator. Although it was not believed to scale to fusion reactor levels, these experiments demonstrated a neutron yield of 2 x 1010 neutrons/second on table-top experiments that could be powered from ordinary laboratory circuits (1 kilowatt). The basis for scaling the output up to 1x1011 n/s has been established. In addition, IEC devices have run for cumulative times approaching 10,000 hours. The essential features of the IEC plasma neutron source, compared to existing sources of the same cost, size and power consumption, are: neutron yield of 1011 compared to 108, lifetime of 10,000 hours compared to 500, and operation is pulsed or steady state compared to pulsed. The design of a conventional IEC source is a spherical vacuum chamber containing a spherical grid. The grid is raised to a high negative potential. A breakdown develops between the chamber wall and the grid, and this plasma becomes a source of positive deuterium and tritium ions. These ions are accelerated to the center of the vacuum chamber sphere where they may collide. If the grid is raised to a nominal 100 kV, the coulomb barrier for D-T fusion, then the fusion cross section becomes quite large and the neutron production proceeds. The limiting factor has been high densities associated with the Paschen breakdown curve. Because of the high densities, the ions tend to collide multiple times before reaching the center and do not collide with the full accelerating potential. The Los Alamos IEC uses a triple grid design. In the triple grid IEC device, the inner grid is the accelerating grid and serves the same function as the single grid in conventional IEC systems. The central grid serves as electrical isolation, and is held at ground potential. The outer grid is raised to a modest positive potential, say 200 volts. Dispenser cathodes around the vacuum chamber wall inject electrons. The electrons are trapped and orbit around the outer grid, ionizing a local plasma. Because of the modest potential, the breakdown occurs at a different point on the Paschen curve, at a much lower density. The limit is further relaxed by the injected ionization from the dispenser cathodes. The result is a lower density plasma. The result is a tight focus of fully accelerated ions that collide in a beam-beam mode. The collision energy and neutron yield are large.
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Development of a high-efficiency neutron counter using novel materials
Neutron detection efficiency is an important figure of merit for waste assay applications that must measure small quantities of material. It is also important for neutron coincidence counting and multiplicity counting because the detection of double- and triple-correlated events scales as the detection efficiency squared and cubed, respectively. Conventional thermal neutron detection systems typically employ {sup 3}He detector tubes embedded in polyethylene. The polyethylene moderates the neutrons so they can be detected by the {sup 3}He tubes. However, the hydrogen in the moderator also absorbs neutrons and reduces the diffusion length. We have extensively explored alternate designs that use both polyethylene and other industrial plastics with lower concentrations of hydrogen. In MCNP studies, we have achieved higher detection efficiency by using both polyethylene and other plastics in a hybrid design. In this paper we will present the design of a nominal 30%-efficient, 200-liter neutron counter that uses a hybrid design. We will show comparison results of this design compared to a standard polyethylene design. Finally, this counter has been constructed and tested, and was used in the Los Alamos waste assay course. We will show comparisons of the experimental results from this counter to the MCNP predictions
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Passive equilibrium studies on ZT-P
The poloidal field system of ZT-P was modified by the addition of a transformer, which coupled the magnetizing and equilibrium circuits. ZT-P is a small, air core, Reversed Field Pinch, operated at the Los Alamos National Laboratory. It is used as test bed for the much larger ZT-H, RFP experiment, now under construction at LANL. Planned experiments include size scaling measurements and determining the effect of low time constant measurements and determining the effect of low time constant shell operation. ZT-P has had an entirely passive equilibrium system, which did not provide a well centered equilibrium, although a tolerable equilibrium was realized by removing half of the equilibrium coil set. The transformer was added to the poloidal field system to adjust the equilibrium current for a centered plasma, while using the entire coil set. It also had the effect of reducing the dependence of the equilibrium on the plasma resistance. Stable, well centered discharges were achieved over a broad range of plasma currents. The improved equilibrium also lowered the loop voltage and extended the discharge lifetime. These experiments also investigated the unique problems of equilibrium systems on air core RFP devices. 26 refs., 6 figs
Endotoxin, ammonia, and total and respirable dust in swine confinement buildings: the effect of recirculated air and respiratory protective masks
Caretakers and pigs in dusty environments
with particles and toxic gases may sustain
health consequences. We studied concentrations of ammonia, endotoxin, and total and respirable dust particles in four mechanically ventilated swine nurseries and two grower facilities using an ammonia sampler, filter, and British cyclone. In two of the nursery facilities, we determined the protection offered by respiratory masks that were mounted on glass funnels with filters or
British cyclones and sampled for dust. In
response to the increasing summer ventilation, large, nonrespirable particle concentrations in swine building atmospheres were reduced more completely by ventilation air movement than smaller respirable particles or ammonia. Total airborne endotoxin concentrations were similar to those eliciting pulmonary reactions. Total airborne endotoxin correlated with total suspended particles rather than respirable particles. Smaller respirable fecal particles enriched in endotoxin apparently stick to larger nonrespirable particles or are agglomerated before they became airborne. Internal recirculated air partially limited the mass concentration of
respirable particles in the breathing zone of
swine caretakers at lower but not higher
ventilation rates. Respiratory protection
limited the potential total dust exposures of
swine caretakers in such atmospheres (<25 %, 2-tie masks; <50%, I-tie masks of the total suspended particles). Respirable particles were reduced to <55% by 2-tie masks. Properly worn 2-tie masks protect against both large and small respirable particles in swine confinement facilities